Recently, the development of oil wells and gas wells in deep layer has been advanced. (hereinafter, an oil well and a gas well are collectively referred simply to as “an oil well”. Also herein, “a stainless steel for oil well” includes a stainless steel for oil well and a stainless steel for gas well, and “a stainless steel pipe for oil well” includes a stainless steel pipe for oil well and a stainless steel pipe for gas well.) A deep oil well has a high-temperature environment. “A high-temperature environment” contains carbon dioxide gas or carbon dioxide gas and hydrogen sulfide gas, which are corrosive gases. The term “a high temperature” as used herein represents a temperature not lower than 150 degrees C. The oil well pipe used in a high-temperature environment of deep oil well is required to meet the three requirements as below.
(1) High strength. Specifically, the 0.2% offset yield stress is 758 MPa or higher (110 ksi class or higher). For the deep oil well, since the well has a large well depth, the length and weight of steel pipe used increase. Therefore, a high strength is required.
(2) Excellent corrosion resistance. Specifically, the corrosion rate in a high-temperature environment is lower than 0.1 g/(m2·hr). Further, the oil well pipe is less liable to crack even when the pipe is stressed. That is, the oil well pipe has excellent stress corrosion cracking resistance. Hereinafter, “stress corrosion cracking” is also abbreviated as SCC. When reference is made to “excellent corrosion resistance in high-temperature environment” herein, it means that the corrosion rate is low, and the SCC resistance is excellent.
(3) Excellent sulfide stress corrosion cracking resistance at normal temperature. In the case where the stainless steel pipe for oil well is used for a production well, a fluid (oil or gas) produced from the oil well in high-temperature environment flows in the stainless steel pipe. When the production of fluid from the oil well stops for some reason, the temperature of the fluid in the stainless steel pipe near the earth's surface decreases to the normal temperature. At this time, sulfide stress corrosion cracking (hereinafter, also abbreviated as SSC) may occur in the stainless steel pipe that is in contact with the normal-temperature fluid. Therefore, the stainless steel pipe for oil well is required to have not only SCC resistance at high temperatures but also SSC resistance at normal temperature.
JP2005-336595A (hereinafter, referred to as Patent Document 1), JP2006-16637A (hereinafter, referred to as Patent Document 2), and JP2007-332442A (hereinafter, referred to as Patent Document 3) have proposed stainless steels for the use in high-temperature environments. In improving the corrosion resistance at high-temperature environments, chromium (Cr) is effective. Therefore, the stainless steels disclosed in Patent Documents 1 to 3 contain much Cr.
The stainless steel pipe disclosed in Patent Document 1 contains 15.5 to 18% of Cr, this Cr content being higher than that of the conventional martensitic stainless steel (the Cr content is 13%). Further, the chemical composition of the stainless steel pipe satisfies the formula of Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≧11.5. Since the chemical composition satisfies this formula, the micro-structure consists of a two-phase micro-structure of ferritic phase and martensitic phase. As a result, the hot workability is improved. Further, the chemical composition of the stainless steel pipe contains Ni and Mo as essential elements and contains Cu as a selective element. Therefore, the corrosion resistance of stainless steel pipe is improved.
The stainless steel pipe disclosed in Patent Document 2 contains 15.5 to 18.5% of Cr. Further, the stainless steel disclosed in Patent Document 2 contains Ni, which improves the corrosion resistance, as an essential element. In the stainless steel pipe disclosed in Patent Document 2, Mo and Cu are selective elements.
The stainless steel pipe disclosed in Patent Document 3 contains 14 to 18% of Cr. The stainless steel pipe disclosed in Patent Document 3 further contains Ni, Mo and Cu. Therefore, the stainless steel pipe is corrosion resistant. Further, the micro-structure of the stainless steel pipe disclosed in Patent Document 3 contains a martensitic phase and an austenitic phase having a volume ratio of 3 to 15%. Therefore, the stainless steel pipe is tough.
As described above, the stainless steels disclosed in Patent Documents 1 to 3 contain more than 13% of Cr. Further, these stainless steels contain alloying elements of Ni, Mo, Cu, etc. as an essential element or a selective element. Therefore, the corrosion rate in high-temperature environments decreases. For example, in the working example of Patent Document 1, a decrease in corrosion rate in high-temperature environments has been proved (refer to Table 2 in Patent Document 1).